Method to bootstrap into a frequency agile ad hoc network of radios that use a non-deterministic frequency pattern

ABSTRACT

A method to facilitate late entry of a late entrant node into an ad-hoc radio network involves accessing from a memory at the late entrant node an initial frequency hopping sequence for the ad-hoc radio network. The late entrant node uses at least the initial frequency hopping sequence to determine a first frequency to monitor during a communication epoch corresponding to one of the frequency hops. The late entrant node thereafter monitors the first frequency to detect the presence of a beacon signal transmitted by a participating node of the network. Subsequently, the late entrant node determines a second frequency included in the current hopping sequence of the network based on information contained in the beacon signal.

BACKGROUND OF THE INVENTION

1. Statement of the Technical Field

The invention concerns ad-hoc radio networks, and more particularly methods to facilitate late entry of nodes into ad hoc radio networks that use frequency hopping sequences that are non-deterministic.

2. Description of the Related Art

Frequency agile communication systems can hop over a sequence of frequencies to avoid detection and/or interference. When such frequency hopping types of communications systems are used to form an ad-hoc network, they must include some way for nodes to enter into the network after frequency hopping has already begun. To this end, frequency hopping networks often use a pre-determined frequency hopping pattern. In such systems, a late entrant into the network can scan one or more known hopping frequencies until it detects a network communication on one of those frequencies. Once such communication is detected, the late entrant to the network can easily achieve hop synchronization with the remainder of the network, since the hopping sequence and timing are known.

Cognitive radio systems comprise radio transceivers that automatically change their transmission or reception parameters in response to a spectral environment. The active control over the parameters of transmission and reception generally relies upon certain cognitive radio activities, such as spectrum monitoring and dynamic selection of transmission frequency among network nodes. This process of dynamic frequency modification is sometimes referred to as dynamic spectrum access or DSA. In such a system, the radio transceivers automatically choose the frequency on which to communicate with each other. The frequency used to communicate is usually selected in such a way as to minimize interference with other users of the same spectrum. For example, the frequency can be selected to avoid interference with a class of wireless spectrum user which has been designated as a primary user of such spectrum.

SUMMARY OF THE INVENTION

Embodiments of the invention concern a method for facilitating late entry of a late entrant node into an ad-hoc radio network. Briefly, the method involves accessing from a memory at the late entrant node an initial frequency hopping sequence for the ad-hoc radio network. The initial frequency hopping sequence specifies two or more frequencies, each respectively assigned to one of a plurality of defined frequency hops of the frequency hopping sequence. The late entrant node uses at least the initial frequency hopping sequence to determine a first frequency to monitor during a communication epoch corresponding to one of the frequency hops. The first frequency is a frequency included in a current hopping sequence of the ad-hoc network, but not included in the initial frequency hopping list. The late entrant node thereafter monitors the first frequency to detect the presence of a signal transmitted by a participating node of the network. Subsequently, the late entrant node determines a second frequency included in the current hopping sequence of the network based on information contained in the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described with reference to the following drawing figures, in which like numerals represent like items throughout the figures, and in which:

FIG. 1 is a block diagram that is useful for understanding a network node of a frequency agile ad hoc network that incorporates dynamic spectrum access methods

FIG. 2, is a diagram that is useful for understanding a frequency agile ad hoc network that incorporates dynamic spectrum access methods.

FIG. 3 is a flow chart that is useful for understanding the invention.

FIG. 4 is graphic illustration of a portion of a frequency spectrum that is useful for understanding the invention.

FIG. 5 is a table including a master list that is useful for understanding the invention.

FIG. 6 is a table including an allowed list that is useful for understanding the invention.

FIG. 7 is a drawing that is useful for understanding a hopping sequence.

FIG. 8 is a drawing that is useful for understanding a set of hop indices.

FIG. 9 is an exemplary hopping list and an array of pointers.

FIG. 10 shows how pointers can be incremented in the array of FIG. 9.

FIG. 11 is a drawing that is useful for understanding a timing associated with a hopping sequence.

DETAILED DESCRIPTION

The present invention is described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operation are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention.

A wireless ad-hoc network according to the present invention includes a plurality of frequency agile network nodes that communicate in accordance with a frequency hopping scheme. The frequency hopping in such network is performed in accordance with a pattern or sequence which is non-deterministic such that a late entering node will not have information concerning the current hopping sequence. An example of such a system could include a network utilizing cognitive radios that automatically change their transmission or reception parameters in response to a detected spectral environment. In such a system, the pattern or sequence of frequency hopping would be substantially dependent upon current and historical aspects of the spectral environment in which the network is situated. Consequently, a late entrant to the network may never achieve hop synchronization with conventional synchronization methods. In order to overcome this deficiency, a system and method is provided to facilitate the quick and efficient discovery of the non-deterministic frequency hopping sequence by a late entrant to the network

Briefly, the method involves facilitating the late entrance of a node into a frequency agile wireless network where the network hopping sequence changes over time in a non-deterministic way. For example, the changes in hopping sequence can be the result of utilizing a dynamic spectrum access (DSA) process involving the various nodes which participate in the network. In such a DSA process, the hopping sequence changes in response to interfering signal sources (which are not predictable). Consequently, the changes in the hopping sequence would be non-deterministic. A problem with such systems is that a current hopping sequence of the network will generally be unknown by a node which is a late entrant to the network.

In order to overcome this limitation, each network node can have information concerning an initial hopping sequence which is preferred by the network. Each node can also have information concerning a table of alternate frequencies for each hop of a hopping sequence. The table specifies the alternate frequencies in an ordered list. Accordingly, the table provides an ordered listing of alternate frequencies that are specified for each hop of the hopping sequence. When a particular frequency in a hopping sequence will no longer be used for a particular hop in the hopping sequence, the ordered listing can be consulted to determine a next alternate frequency that is specified for that hop of the sequence.

In a network where the hopping sequence changes with time in an non-deterministic way, a node which is a late entrant to the network cannot know the hopping sequence the network is currently using. In fact, such a node may never achieve hop synchronization with the network. However, if each node is provided in advance with the information specified above, a late entrant to the network can quickly and efficiently discover the current hopping sequence using the techniques described herein.

In a preferred embodiment, each frequency hop performed by the network coincides with the beginning of a new communication epoch. Each node that is currently participating in the network (and therefore knows the current hopping sequence) can transmit certain information during the communication epoch which is useful for a late entrant to the network. Specifically, each node will transmit (on the current network hop frequency associated with a communication epoch) information relating to a hop frequency that will be used for a future epoch. This information can be transmitted during a time slot allocated for a beacon signal of the communication epoch. In a preferred embodiment, the node will transmit information concerning the frequency of the next hop frequency or the next hop+1 frequency. The choice of which information to send can be determined using a pseudo-random method. This selected information will then be transmitted by that node during the beacon slot for that epoch. The information transmitted by each node also advantageously includes data for determining a time when the particular network hop frequency will be used.

When a late entering node wishes to join the network, it begins the process by monitoring frequencies included in the initial hopping sequence (which information is known to the late entering node). The monitoring technique can vary somewhat depending on the configuration of the network timing. In one embodiment, each time the node fails to detect a beacon signal during a particular hop, the node will increment to the next alternate frequency (for that particular hop). It will be recalled that the next alternate frequency is specified by the ordered list contained in the table of alternate frequencies. If the node is successful during any hop in finding a beacon signal, then the frequency information for the next and next+1 hops is extracted from the beacon signal and recorded. The node can then monitor the next and next+1 hop frequencies to identify additional frequencies in the hopping sequence. This process of monitoring, incrementing, extracting and loading is repeated until the late entering node has recorded the entire hopping sequence. Other monitoring methods are also possible, as will become apparent as the discussion progresses.

For convenience, the inventive arrangements shall be described herein in relation to an exemplary frequency hopping communication system which utilizes network nodes having a DSA capability. In this regard it should be noted that systems which utilize DSA are an example of one type of frequency hopping communication system where the sequence of hopping frequencies will evolve over time in a non-deterministic way. Still, it should be understood that other types of communication systems can also have frequency hopping sequences that vary over time an in a non-deterministic way. Accordingly, the present invention is not limited to systems using cognitive radios and/or DSA techniques. Instead, the invention is useful for any type of communication network where a frequency hopping sequence evolves over time in a non-deterministic way.

FIG. 1 shows a radio 100 that is useful for understanding the invention. The radio 100 can include a central processing unit 112, a user a main memory 116, and a static memory 118, which communicate with each other via system bus 120. Radio 100 can also include a transmitter 124 through which radio frequency signals may be transmitted, and a receiver through which radio frequency signals can be received. According to a preferred embodiment, the radio 100 includes a transceiver in place of the receiver and transmitter, where the transceiver operates as both a transmitter and receiver.

One or more user input devices 104 can be included in the radio 100, such as a keyboard and/or cursor control device. A user output device 104, such as a display screen can be used to communicate information to the user concerning the operations and status of the radio 100. User input devices 104 and user output devices 102 can communicate with the central processing unit through a system bus, but may also be connected by other interface and bus structures. Network interface hardware 114 can facilitate data communication operations with other data processing equipment to which the radio device is connected by wired or wireless means.

A drive unit 106 includes a computer-readable storage medium 110 on which is stored one or more sets of instructions 108 (e.g., software code) configured to implement one or more of the methodologies, procedures, or functions described herein. The instructions 108 can also reside, completely or at least partially, within the main memory 116, the static memory 118, and/or within the central processing unit 112 during execution thereof by the radio 100. The main memory 116 and the central processing unit 112 also can constitute machine-readable media.

The term “computer-readable storage medium” shall be understood to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present disclosure. The term “computer-readable medium” shall accordingly be taken to include, but not be limited to, solid-state memories such as a memory card or other package that houses one or more read-only (non-volatile) memories, random access memories, or other re-writable (volatile) memories; magneto-optical or optical medium such as a disk or tape. Accordingly, the disclosure is considered to include any one or more of a computer-readable medium or a distribution medium, as listed herein and to include recognized equivalents and successor media, in which the software implementations herein are stored. Those skilled in the art will appreciate that the device architecture illustrated in FIG. 1 is one possible example of a radio. However, the invention is not limited in this regard and any other suitable computing device architecture can also be used without limitation.

Dedicated hardware implementations including, but not limited to, application-specific integrated circuits, programmable logic arrays, and other hardware devices can likewise be constructed to implement the methods described herein. Applications that can include the apparatus and systems of various embodiments broadly include a variety of electronic and computer systems. Some embodiments implement functions in two or more specific interconnected hardware modules or devices with related control and data signals communicated between and through the modules, or as portions of an application-specific integrated circuit. Thus, the exemplary system is applicable to software, firmware, and hardware implementations.

In accordance with various embodiments of the present invention, the methods described below are stored as software programs in a computer-readable storage medium and are configured for running on a computer processor. Furthermore, software implementations can include, but are not limited to, distributed processing, component/object distributed processing, parallel processing, virtual machine processing, which can also be constructed to implement the methods described herein.

Turning now to FIG. 2, there is shown a schematic representation of a communication system in accordance with some embodiments of the present disclosure. The communications system 200 can include one or more communication devices 202 which can include a radio 204. The radios 204 can have an architecture similar to the radio 100 described herein with respect to FIG. 1. The radios 204 can include all or some of the functionality of radio 100. Accordingly, the radio 204 can include a central processing unit (CPU) or processor 206 arranged in communication with a receiver 208 and a transmitter 210. Alternatively, a transceiver can be used in place of the receiver/transmitter combination.

The processor 206 can be programmed with a set of instructions for carrying out the methods and processes described herein for implementing an ad-hoc network. The processor can also be programmed to receive information concerning an RF environment, including for example spectrum sensing information. More particularly, the processor 206 is advantageously programmed to control the operation of receiver 208 to detect the presence of primary users or other forms of interference operating within a range of frequencies. This information can then be used by the processor 206 to classify various frequencies as either clear or occupied. A clear frequency is one where RF transmissions associated with a primary user or another source of interference are of negligible signal strength as compared to some predetermined threshold, or otherwise are not present. An occupied frequency is one that does contain RF transmission signals associated with a primary user or with a source of interference at levels that exceed some predetermined threshold signal level. Accordingly, the radio 204 can determine which portions of the RF spectrum contain frequencies that are currently occupied by primary users, and which frequencies are available for RF communications. Techniques for evaluating the presence of primary users or potentially interfering signals are well known in the art and therefore will not be described here in detail. It should be understood that any suitable techniques or methods can be used for such spectrum analysis, regardless of whether such methods are known now or known in the future.

In the communication system 200, radios 204 are network nodes that communicate with one another to form an ad hoc wireless network. In an embodiment of the invention, the two or more radios 204 automatically change their transmission and reception frequencies in response to a spectral environment. This technique is commonly referred to as dynamic spectrum access (“DSA”). DSA type communications system are well known in the art, but present special design challenges. In order to appreciate these design challenges, it is helpful to understand several problems that arise in such systems. The term “rendezvous” refers to the ability of two or more radios to meet and establish a link on a common channel. When radios dynamically choose the frequency as when using DSA, neither the frequency nor the time at which to rendezvous is known. Opportunistic secondary DSA radios that wish to join an existing network potentially have a large number of potential frequencies on which to operate, and primary users and interference sources may be operating in a number of the potential frequencies.

When DSA is further applied to a network of ad hoc radios, additional problems arise. If an operating network has established that a certain frequency is available for use by the network, then the nodes in the network are able to use that frequency. If an ad hoc network is a single frequency network, existing radios in the network can agree that the current frequency is available. However, this use of the frequency may prevent the network from being joined by a new node that by sensing has determined that the very same frequency is actually being used by a primary user. In such a scenario, the new node is excluded. This scenario is referred to herein as “exclusion.”

In the same operational case, where a single frequency DSA network is established, and one radio detects that a primary user has begun using the frequency that the network is using, a dilemma occurs. The radio may by policy not be allowed to transmit on the frequency of the network, and it cannot be sure that other nodes in the network have detected the presence of the primary user (because of proximity to the primary user). The radio may have to abandon the existing frequency and choose a new one. This operation may result in its exit from the network or bi-furcating of the network. This problem is referred to herein as the “abandonment” problem. Finally, in a single frequency network it may be difficult to perform effective “assessment” of interference or existence of a primary user. In other words, the radios in the network may have trouble monitoring for the existence of a primary user on the channel if the channel is already heavily used by the network. This is especially true if the data traffic that the ad hoc network is transmitting is heavy.

In order to avoid these problems, the radios 204 in communications system 200 are preferably frequency agile devices which communicate with one another using a plurality of frequencies. The radios 204 are synchronized so as to change frequency or “hop” in unison from one frequency to another, whereby the transmission and reception frequency are changed on a regular basis in each radio. Notably, the exclusion, abandonment, and assessment problems are effectively solved by the frequency agile or hopping solution. When the nodes in the ad hoc network observe different spectral environments, they will still with high probability identify a partly overlapping set of available hop frequencies on which they can rendezvous. Since there are almost always at least some overlapping hop frequencies, the “exclusion” problem is effectively solved. Also, when some nodes in the network observe primary users (or other interference) on one of the hop frequencies, they can immediately abandon that frequency, and can with high reliability inform other nodes on a different hop frequency. This enables cohesion of the network; nodes do not have to drop out of the network and established networks will not bifurcate when only a subset of nodes in the network detect changes in the spectral environment. This effectively solves the abandonment problem. Finally, the “assessment” problem is solved since nodes in the network routinely change hop frequencies, channel assessment can be scheduled at a time and on a frequency where it is known a priori that no traffic from the nodes own ad hoc network will occur.

According to a preferred embodiment, a relatively small set of frequencies are used in an active frequency hopping list for the frequency agility solution. This enables relatively fast rendezvous among nodes since there are fewer frequencies that must be monitored by a node attempting to discover an existing network hopping pattern. The method further establishes a priority list of the active frequencies, so that late net enterers or merging networks have a very high probability of identifying the frequency used among each one of the active frequencies. The algorithm for selecting frequencies has the property that a change of state of one frequency (i.e. frequency marked good at one node and bad at another) can at most affect the active list (operating frequencies) at only one node. The method described herein is also advantageous because it facilitates time aligning an epoch associated with the hopping sequence to a universal time source (such as a stable internal reference or GPS). Time aligning the epoch to a universal time source further simplifies and facilitates rendezvous. However, it should be appreciated that the method can operate with or without the need for such a time reference.

Referring now to FIG. 3 there is shown a flowchart that is useful for understanding the present invention. The flowchart illustrates a process that will be separately implemented in each node (i.e., each radio 204) that is part of the ad hoc network of communication system 200. The process begins in step 302 and continues with step 304, in which the node accesses a Master List of all RF frequencies that the radio 204 can physically use. The frequencies in the Master List are preferably separated by some predefined granularity or frequency spacing that is appropriate for the network. For example, the frequency granularity can be Fgran=500 kHz. Still, the invention is not limited in this regard and other frequency granularity values can also be used. The Master List of frequencies generated in step 304 can include all available frequencies within an operating frequency range over which a particular radio 204 can potentially communicate.

For example, in the frequency spectrum chart shown in FIG. 4, the frequencies over which the radio 204 can operate will range from f₁ to f_(c). The frequencies available include frequencies f₁, f₂, f₃, . . . f_(Nall−1), f_(Nall), f_(Nall+1), f_(Nall+2), . . . f_(c-1), f_(c). These frequencies are spaced apart in accordance with the frequency granularity as specified above. Applying step 304 to the frequency spectrum chart in FIG. 4 yields a Master List 500 containing these frequencies as shown in FIG. 5.

In step 306, the authorization to make use of one or more frequencies on the Master List is specified by marking each frequency as “allowed” or “not allowed.” Authorization is determined based on a policy and/or known local users operating on one or more of RF frequencies f₁, f₂, f₃, . . . f_(Nall−1), f_(Nall), f_(Nall+1), f_(Nall+2), . . . f_(c-1), f_(c). For purposes of illustration, we assume that in the spectrum chart of FIG. 4, the frequencies f_(Nall+1), f_(Nall+2), . . . f_(c-1), f_(c) are “not allowed” due to the existence of one or more known local users 402. The Master List in FIG. 5 is marked with an authorization 502 in accordance with step 306 to show that frequencies f_(Nall+1), f_(Nall+2), . . . f_(c-1), f_(c) are “not allowed.” The policy used to mark the Master List can be any policy which specifies certain frequencies that are not permitted to be used by the communication system 200. The policy can be stored in a suitable memory location, such as main memory 116 in FIG. 1. Similarly, a list of known local users can be determined by any suitable means. For example, in an embodiment of the invention, information concerning known local users of frequencies from f₁ to f_(c) can be pre-determined and stored in a memory location, such a memory location contained in main memory 116. Alternatively, the information concerning known local users can be determined by spectrum monitoring performed by one or more radio 204.

In step 308, an Allowed List is created by making a deterministic list of all the index numbers assigned to the frequencies that have been marked “allowed” in step 306. Note that the index values referred to herein are those subscript values assigned to allowed frequencies in the Master List of FIG. 5. Accordingly, for RF frequencies f₁, f₂, f₃, . . . f_(Nall−1), f_(Nall) contained in the Master List, the corresponding index values used in the Allowed List would be 1, 2, 3, . . . Nall−1, and Nall; where Nall represents the total number of frequencies on the Allowed List. An Allowed List 600 is shown in FIG. 6. For convenience in understanding the invention, an RF frequency list 601 containing an RF frequency corresponding to each index value is shown in a position adjacent to corresponding index value. The ordering of the index values in the Allowed List is created by a deterministic selection process. Accordingly, the resulting index values 1, 2, 3, . . . Nall−1, and Nall contained on the Allowed List in FIG. 6 are shown in deterministic order. For example, the deterministic list can be created using a pseudorandom process. Pseudorandom selection processes are well known and therefore will not be described here in detail. However, it should be appreciated that any suitable pseudorandom process can be used for selecting a deterministic order of the index values contained in the Allowed List and the invention is not limited in this regard.

In step 310, all of the index values entered on the Allowed List 600 are initialized in a status list 602 as “clear” frequencies. A “clear” status indication means that a frequency corresponding to that particular index value is not occupied by a primary user and is therefore available for use. A set of pointers 604 can be used to point to particular frequency index values contained in the Allowed List 600. The purpose of the pointers will become more apparent as the description of the invention progresses.

Steps 304-310 can be performed in an automated way at each node based on stored data concerning known local users or other communications policies in effect. However, these steps can also be performed by a technician responsible for radio setup and initialization. For example, these steps can performed at a communications facility prior to deployment of each radio. Moreover, in some embodiments, the allowed frequencies can be assigned a rank value which specifies an order of preference among the allowed channels. In such embodiments, higher ranking frequencies which are more preferred will be given a higher preference for use in frequency hopping operations as compared to allowed frequencies having a lower rank. These and other aspects of the invention will be presented in further detail below.

In the present invention, the radios 204 which comprise each node of the communication system 200 are frequency agile, meaning that they hop in unison from one frequency to the next. The communication frequency used by the ad hoc network of radios is constantly changing with each hop. This concept is illustrated in FIG. 7, which shows that the nodes of the communication system 200 can communicate for some predetermined period of time at a particular RF frequency during at HOP[0], transition to a different RF frequency at HOP[1], and transition to another frequency at HOP[2]. The process continues this way, changing frequency with each hop for some predetermined number of hops, before returning to the Hop[0], at which point the hoping sequence is repeated. As will be understood from FIG. 7, Nhop is some value which represents the number of frequencies that the radio can hop over at a given time. The value of Nhop can be determined in a variety of ways. For example, in some embodiments, the value of Nhop can be pre-set as part of a system design. In other embodiments, the value of Nhop can be determined dynamically. For example, the number of hop frequencies in the sequence can be determined by the number of frequencies available, the existence of interference sources or jammers, and so on.

In step 312, the process continues by creating a total of Nhop pointers into the allowed list. One pointer value is assigned for each frequency hop to specify the frequency to be used during that hop. A set of pointers 800 ₁, 800 ₂, 800 ₃, . . . 800 _(Nhop) is illustrated in FIG. 8. Each pointer 800 ₁, 800 ₂, 800 ₃, . . . 800 _(Nhop) has a value which points to one of the frequencies on the Allowed List 600 in FIG. 6. For example, the pointer for Hop[0] in FIG. 8 is shown to have a value of 2. It can be observed that pointer value 2 points to frequency f₄ in FIG. 6. Accordingly, the frequency assigned to Hop[0] is f₄. Similarly, the pointer for Hop[1] is shown to have a value 0, which points to RF frequency f₂ in FIG. 6. Accordingly, the frequency assigned to Hop[1] in this example is f₂. Each hop thus has a pointer which references a particular RF frequency for that hop. According to a preferred embodiment, each of the pointers is set to some initial value, but this value can change dynamically during a hopping session in accordance with a dynamic frequency assignment scheme.

According to one embodiment, the initial values for the pointers in FIG. 8 can be determined by based on the integer part of the sequence:

0,*Nall/Nhop, 2*Nall/Nhop, 3*Nall/Nhop, . . . (Nhop−1)*Nall/Nhop

where Nall represents the total number of frequencies on the allowed list, and Nhop represents the number of frequencies the radios can hop over in a hopping sequence. The foregoing concept is illustrated in FIG. 9 which shows an exemplary table 900 that is similar to the table described in FIG. 6. Accordingly, table 900 includes Allowed List 904, and a list 908 of frequencies corresponding to each index on the Allowed List. The Allowed List 904 is a deterministic list of index values corresponding (in this example) to 28 allowed frequencies (Nall=28). In FIG. 9, the value of Nhop is assumed to be 7, meaning that there are a total of 7 hop frequencies that are used in a hopping sequence. Using the values Nall=28 and Nhop−=7 in the sequence stated above, the initial values for the pointers would be 0, 4, 8, 12, 16, 20, and 24. These initial values 906 are arranged in a tabular format in an array 902. As may be observed in array 902, the remainders of the index values not used as initial values are also contained in the array, and are arranged in sequence in columns 914 corresponding to each hop. In some embodiments of the invention, pointers can be initially set only to allowed frequencies having a higher order of preference as compared to other frequencies on the allowed list.

The pointers in array 902 point to index values in the Allowed List 904. The pointers specify which one of the allowed frequencies will be used for each hop. In the example shown in FIG. 9, the pointer for Hop[0] has an initial value of 0, which points to frequency index value 19 in the Allowed List 904. Accordingly, the RF frequency assigned to Hop[0] is f₁₉. Hop[1] has an initial value of 4, which points to frequency index value 22 in the Allowed List 904. Accordingly, the RF frequency assigned to Hop[1] is f₂₂. Hop[2] has an initial value of 8, which points to frequency index value 11 in the Allowed List 904. Accordingly, the RF frequency assigned to Hop[2] is f₁₁. Each hop is assigned an initial frequency in this way.

In step 314, a function HOPtod (time of day) can choose which hop index points to the frequency of the hop used at the particular time of day. Any suitable function can be used for this purpose. According to some embodiments, the time of day information can be provided by a satellite signal, such as a GPS time signal. The HOPtod function coordinates the timing of each node and ensures that each node is communicating using the correct hop index at correct time. The HOPtod function will change the hop index at each node after each frequency hop.

In step 316, with the frequency for each hop having been established in the foregoing steps, the radios 204 of the communication system 200 can begin communicating using the frequency hopping sequence specified by the initial values 906. However, at any time during a frequency hopping communication session, a primary user or some other source of RF interference may begin using an allowed frequency within the frequency spectrum 400 shown in FIG. 4. Accordingly, each radio 204 can periodically monitor the frequencies in the hopping sequence to determine whether the status of such channels is still “clear”. If the channel is not clear (318:No) then in step 320, the radio 204 will change the status 910 for that frequency from “clear” to “occupied”. Thereafter, in step 322, the radio will increment the pointer for HOP[i] to the next pointer on the list until a current index for HOP[i] is one that is marked “clear”.

An example of the frequency incrementing process will now be described with reference to FIG. 9. For this example, we note that in array 902, Hop[3] has an initial pointer value of 12. With reference to table 900, it can be observed that index value 12 corresponds to frequency f₁₆. Assume that it is determined that RF frequency f₁₆ is occupied. In such event, the pointer for Hop[3], will need to be incremented to the next available pointer value. Accordingly, the initial pointer value for HOP[3] is incremented from 12 to 13. If the RF frequency corresponding to pointer 13 is also occupied, then the pointer value can be incremented again to pointer 14. For purposes of this example, assume that pointer 14 is “clear” so the new frequency for Hop[3] will be f₈ as per table 900.

This incrementing process is further illustrated in FIG. 10. The arrows in FIG. 10 show that pointer values are incremented in for each hop until a clear channel is located. Accordingly, in the case of Hop[3] described above, if a status 910 of the RF frequencies corresponding to pointers 13, 14, and 15 are all marked “occupied” (as opposed to “clear”) then the pointer can continue incrementing from column 1014 a, into the adjacent column 1014 b. Thus, pointers 16, 17, and 18 can be checked to see if they correspond to frequencies status 910 marked “occupied” or “clear”. For example, if pointer value 18 is marked “clear” then that pointer can be assigned to Hop[3] and the system can resume the hopping sequence. Note with the foregoing dynamic frequency assignment method, changing one of the hop frequencies does not generally affect the frequencies assigned to the other hops in a sequence. It can also be understood from FIG. 10 that the incrementing will roll over to pointer zero (0) when incrementing beyond the final value in the sequence (Nall−1), which in this case is 27.

In some embodiments, the incrementing process can also involve consideration of the relative preference among available RF frequencies. For example, if a particular pointer is incremented to a clear channel, but the clear channel has a relatively low rank or (i.e., the frequency is not preferred), then the pointer value can be further incremented until a clear channel is identified with a higher rank or preference for use. The system can increment through the entire list to search for clear channels with higher rank or can select a clear channel with highest rank from among the first two or three clear frequencies that are available.

The frequency agile network described in FIGS. 1-10 has several advantages over conventional arrangements. An ad hoc network using frequency hopping and dynamic spectrum access (DSA) methods as described herein will allow greater opportunities for radio terminals 204 to communicate with the network when some frequencies local to the particular radio are jammed. Hopping also allows for the evaluation of the frequencies that are in the current hop sequence because none of the frequencies are used continuously. In other words, there will always be times when each frequency is not being used by the network, and therefore its use by other services can be monitored. Another advantage is that the network is highly stable since it uses a list of allowed frequencies that are essentially fixed.

In an embodiment of the invention, an ad hoc network described in relation to FIGS. 1-10 will hop at some epoch rate. For example, the epoch rate can be 135 milliseconds in length. FIG. 11 shows a hopping sequence having an epoch 1102 which has an epoch rate of 135 milliseconds. During each epoch, nodes will communicate with each other using an RF frequency assigned for that particular hop in accordance with the dynamic frequency assignment methods described herein. Each epoch 1102 can include certain time periods or time slots which are designated as beacon slots 1104. Each node (e.g. a node defined by a cognitive radio 204) will be assigned a beacon slot. In some embodiments of the invention a beacon slot can be a relatively small portion of the overall epoch. For example, each beacon slot can be about 2 milliseconds in duration, and can comprise about 192 bits. Still, it should be appreciated that the invention is not limited to the timing or bit lengths stated herein. Each node is assigned one of the beacon slots 1104, during which time the node can communicate certain information relevant for communicating. For example, the node can communicate information concerning nodes that they intend to communicate with, transmission data rates, a time slot within the epoch that they will use to transmit, the nodes they are communicating with and so on.

Significantly, nodes can also communicate during the beacon slot 1104 certain information concerning occupied frequencies. In particular, the various nodes can share the spectrum sensing information they have collected regarding spectral conditions at their location. The nodes can further communicate during the beacon slot 1104 an index number for a frequency that they intend to use for their next transmission. This information can be used by nodes which are late entrants to the network for purposes of synchronizing their hopping frequencies with the hopping frequencies actually in use by the network. In this regard it must be understood that all nodes within the network are configured to generate their deterministic Allowed List 904 using the same methods as other nodes of the network. Consequently, the Allowed List 904 that is generated and used for each node in the network should be identical. Likewise, each node will generate the same array 902, with the same initial index values 906. Each node will utilize the same set of hopping frequencies, and if a hopping frequency should change, each node will transition to a new frequency for that hop in the same way by incrementing through the index values in array 902.

Whenever a terminal finds a frequency that is in use by a primary user, it will transmit (during its beacon slot 1102) the index of that frequency, together with a tag which specifies that the frequency corresponding to that index is “occupied”. Any node receiving this information will retransmit the “occupied” message for that index during its own beacon slot 1102, until such time as the node hears all of its neighbors send the same message. This process ensures that information concerning occupied frequencies is rapidly disseminated to all nodes in the network.

By utilizing the methods described herein, frequency hopping will occur over a relatively small list of frequencies contained on an Active List of frequencies. As will be appreciated from the discussion relating to FIGS. 3-11, this list is actually an array of pointers into the larger list called the Allowed List 904. All members of the network have the same Allowed List, in the same order; which does not change. The Active List will start out as a set of initial values 906 but will change over time. Still, the Active List should be the same for all nodes except for transient conditions that occur when a primary user is detected, and a hop frequency is changed. Even then, the Active List for each node should be mostly the same.

Whenever a node has time when it is not scheduled to receive or transmit, it will scan frequencies for primary users. Both the Active List and candidate frequencies are scanned. Candidate frequencies can include any of the frequencies in the Allowed List. Whenever a node detects a primary user on a frequency, it communicates the index value for the frequency together with a tag which marks the frequency as “occupied”. This tag and index then flood the network. Detection of a primary user or reception of an occupied index causes a node to immediately abandon the frequency unless the network is currently operating on a single frequency. This is an important advantage insofar as it ensures that any node can cause the entire network to discontinue use of an occupied frequency. Moreover, the process of abandoning occupied frequencies begins immediately when a primary user has been detected, thereby providing practically instantaneous abandonment of occupied frequencies. Upon abandonment of a frequency, the next candidate frequency is used in its place. A mechanism for recovery can be provided for retesting frequencies that have previously been marked ‘occupied’ since this state could change due to the passing of time and change of location of the network.

Each node will respond to an “occupied” frequency message as if it had detected the occupied frequency itself. In other words, it will execute the pointer incrementing process described above in relation to FIG. 10. Should a first node detect an occupied frequency during the same hop as it receives a notification of a different occupied frequency from a second node, then the first node can send its own notification concerning the occupied frequency first, and can queue the notification from the second node. If a node receives multiple notifications of different occupied frequencies during the same hop, then it can randomly select which notification to re-transmit first, and can queue the other notification(s) for re-transmission during a subsequent epoch.

When there are no messages to be sent concerning “occupied” frequencies, a node will send either the frequency to be used during the next hop of the network or the frequency to be used for the hop after that (next hop+1). The choice between the next and after-next frequencies can be made randomly and independently by the nodes. These frequencies are sent to facilitate net synchronization for nodes that are late entrants into the network, or for any other nodes that may have somehow become partly out of synchronization with the remainder of the net.

Recovery of an “occupied” frequency can be accomplished by any suitable means. For example, in step 320, when an Allowed List index entry is changed from “clear” to “occupied,” the entry can also be marked with a time stamp which indicates the time when the change occurred. Thereafter, in step 322 whenever a Hop[i] index at a node is incremented to a frequency that is listed as “occupied,” it accesses the time stamp, adds to this time stamp value a persistence value, and compares the result to the current time. If the value is less than the present time, the node can schedule a scan of the frequency corresponding to that particular Allowed List index. If the value is greater than the present time, then the node takes no action with regard to that particular Allowed List entry, and simply increments on to the next frequency. In the case where a scan of the frequency is scheduled, the node will perform a scan of the frequency at the earliest opportunity corresponding to the scan schedule. At that time, a scan of the frequency will permit the node to determine if the frequency is locally unoccupied. If so, the radio will broadcast a request to the network for recovery of that frequency which is presently marked “occupied”. Every node that receives the recovery message will schedule a scan of the frequency and will forward the result to a net master node. If the net master node receives unanimous consensus from the network that the frequency is no longer occupied, it sends a message with a down count value. The down count value is decremented with each new frequency hop, regardless of whether the message is stationary or in transit from one node to another. On the hop when the down count value reaches zero, the frequency which was previously marked “occupied” is marked as “clear” for the next appropriate hop.

A late net entry node is a node that attempts to enter the network at some time after the network has already been established. It is anticipated that such network will start out with a set of initial pointer values 906, but over time will have migrated to pointer values that are different from those initial values in accordance with steps 318, 320, 322 as previously described. A late net entry node in such a scenario will also start out with the set of initial pointer values 906, but will lack information concerning how the pointer values have changed over the course of time. Accordingly, the late net entry node must have some process by which it can discover the hop frequencies that are currently in use by the network. This process will now be described in further detail.

An ad-hoc network according to the inventive arrangements will have some initial frequency hopping sequence that is well defined by initial pointer values 906. This initial frequency hopping sequence is advantageously stored at each node and can be accessed when needed. If the ad-hoc network has just begun operating, the initial frequency hopping sequence may be the same as the frequency hopping sequence that is currently in use (i.e., the current frequency hopping sequence). Often, however, the current frequency hopping sequence will be different as compared to the initial frequency hopping sequence due to frequency migration by the network. Still, the late entrant node can use the initial frequency hopping sequence to help identify which frequencies are included in the frequency hopping sequence that is currently in use by the network (current frequency hopping sequence).

The late entrant node begins the process of identifying the current frequency hopping sequence by accessing from memory the initial pointer values 906. The late entrant node uses the initial frequency hopping sequence to determine one or more hop frequencies to begin monitoring. The late entrant node will then begin monitoring one or more of the frequencies corresponding to the initial frequency hopping sequence. The manner in which these initial frequencies are monitored is described in further detail below. In general, however, if the late entrant node fails to detect network transmission on a monitored frequency, it will consult an ordered list of alternate frequencies which can be used during that particular frequency hop. In the exemplary embodiments described herein, this ordered list is contained in array 902.

By iteratively monitoring frequencies on the ordered list, the late entrant node eventually detects a network transmission, thereby discovering a first frequency included in a current frequency hopping sequence of the ad-hoc network. The first frequency can be a frequency assigned to any frequency hop Hop[i] in a current frequency hopping sequence of the network. Once this first frequency is discovered, the late entrant node can continue to monitor that frequency to receive a beacon signal. The beacon signal is transmitted by a participating node of the network during a beacon slots 1104. The beacon signal is transmitted at the first frequency during a communication epoch 1102 corresponding to a particular frequency hop. As a result of such monitoring, the late entrant node determines a second frequency included in the current hopping sequence of the ad-hoc network. Specifically, the second frequency is determined based on information contained in the beacon signal.

From the foregoing, it will be appreciated that the first frequency discovered by the late entrant node will often not be a frequency included in the initial frequency hopping sequence as specified by initial pointer values 906. This deviation from the initial frequency hopping sequence is due to network migration away from the initial frequency hopping sequence over some period of time that the network has been operating. Once the network has begun such migration, the first frequency noted above is determined by using the initial frequency hopping sequence and the ordered list contained in array 902. Specifically, by referring to each pointer in the array 902, and the corresponding list 908 of allowed frequencies, the late entrant node can determine alternate frequencies to monitor for each hop. Thus, the late entrant node monitors a frequency on the initial frequency hopping list for a defined frequency hop and, if the beacon signal is not detected, continues on to monitor at least one of the alternate frequencies for the defined frequency hop. The alternate frequencies are iteratively monitored in a sequential order defined by the ordered list until such time that the beacon signal is detected.

As an example, consider the case where the late entrant node begins by monitoring a frequency f₁₁ corresponding to pointer value 8 in array 902. If no network transmissions are detected for some period of time (discussed below), the late entrant node will end monitoring of f₁₁ and instead begin monitoring frequency f₂ corresponding to pointer value 9. If no signal is detected on frequency f₂ after some period of time, then the late entrant node can continue on to monitor frequency f₉ corresponding to pointer value 10. The process continues until the late entrant node discovers a frequency that is included in the current frequency hopping sequence for the network.

In a preferred embodiment, nodes participating in the network will transmit as part of the beacon signal the frequency for a future hop in the frequency hopping sequence. For example, the future hop can be the next hop in the frequency hopping sequence, or the next+1 hop. The particular frequency information that is transmitted can be chosen by a pseudorandom selection process. Accordingly, the late entrant node can continue to monitor the first frequency after receiving information about the second frequency included in the current hopping sequence. Such monitoring can continue, for example, during future communication epochs 1102 until a beacon signal is received (on the first frequency) that contains information about a third frequency included in said current hopping sequence of the network. The late entrant node can use the information it has obtained to perform further monitoring during additional hops included in the current hopping sequence for the network. For example, the late entrant node can monitor the second frequency and/or the third frequency to discover additional information concerning frequencies to be used for future hops.

The procedure described above for monitoring frequencies in the initial frequency hopping list will vary to some degree depending on the network timing configuration. In a first scenario, it is assumed that the network relies on a time reference, such as a time clock signal received from a global positioning system (GPS). In such an embodiment, the timing of each epoch 1102 and associated beacon time slots 1104 will be determined by a GPS time of day. The late net entry node will monitor each of the initial Nhop frequencies specified by initial pointer values 906 to look for beacon signals during beacon time slots 1104. The nodes will use the GPS time of day (TOD) information to determine the time during which the beacon transmissions should be present. For each Hop[i], the node will monitor the frequency corresponding to an initial pointer value 906. Each time the node fails to detect a beacon signal during a particular Hop[i], the node will increment to the next pointer value (e.g. in array 902) for that particular Hop[i]. If the node is successful during any hop in finding a beacon, then the next and next+1 information is extracted from the beacon signal, if available, and loaded into the next and next+1 Hop[i] pointers. This process of monitoring, incrementing, extracting and loading is repeated until all Hop[i] indices point to frequencies with valid beacons.

In a second scenario, it is assumed that the network does not rely upon a time reference (e.g. a GPS time) for specifying a time of day for the beginning and end time of each Hop[i]. In this scenario, the late net entry node will not know the time of day when each epoch 1102 will begin/end and therefore will not know the time of a beacon time slot 1104. Accordingly, a late net entry node will monitor for beacon signals on each frequency according to an initial set of pointer values 906. The late net entry node will dwell on each frequency for Nhop+1 hop durations. After each dwell, the next hop index is used. Each Hop[i] that fails to find a beacon during the dwell time is incremented to its next value (e.g. following the pattern shown in FIG. 10). If the node is successful during any hop in finding a beacon, then the next and next+1 information is extracted from the beacon signal, if available, and loaded into the next and next+1 Hop[i] pointers. This process of monitoring, incrementing, extracting and loading is repeated until all Hop[i] indices point to frequencies with valid beacons.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents. 

We claim:
 1. A method for facilitating late entry of a late entrant node into an ad-hoc radio network in which a network hopping sequence changes over time in a non-deterministic manner in response to a spectral environment, comprising: accessing from a memory at the late entrant node an initial frequency hopping sequence for said ad-hoc radio network, said initial frequency hopping sequence specifying a plurality of frequencies, each respectively assigned to one of a plurality of defined frequency hops of said frequency hopping sequence, said initial frequency hopping sequence specifying a sequence of frequency hops used by the ad-hoc network at a time corresponding to initiation of the ad-hoc network; at a time after said initiation when the initial frequency hopping sequence has migrated in a non-deterministic way to a current frequency hopping sequence different from the initial frequency hopping sequence, and unknown by the late entrant node, discovering the current frequency hopping sequence by exclusively monitoring at least a first frequency specified by the initial frequency hopping sequence; if the presence of a signal transmitted by a participating node of said ad-hoc network is not detected on at least the first frequency as a result of the exclusive monitoring, iteratively monitoring one or more alternate frequency values in accordance with an ordered list respectively specified for at least the first frequency until detecting the presence of the signal, said signal comprising (1) first information concerning at least one “next+M” hop frequency of the current hopping sequence of said ad-hoc network, where M is an integer having a value equal to or greater than zero, and (2) second information useful for determining a time when the at least one next+M hop frequency will be used by nodes of said ad-hoc network; and determining a second frequency included in said current hopping sequence of said network based on the first information contained in said signal.
 2. The method according to claim 1, further comprising specifying with said ordered list an order in which said alternate frequencies are to be monitored by said late entrant node.
 3. The method according to claim 1, further comprising monitoring at least the first frequency specified in said initial hopping sequence for a defined frequency hop and, if said signal is not detected, then monitoring at least one said alternate frequency for said defined frequency hop.
 4. The method according to claim 3, further comprising monitoring in a sequential order each of said alternate frequencies for said defined frequency hop until said signal is detected.
 5. The method according to claim 4, further comprising specifying said sequential order with said ordered list.
 6. The method according to claim 1, further comprising continuing to monitor said first frequency during at least a second communication epoch, and determining a third frequency included in said current hopping sequence of said network based on information contained in said signal.
 7. The method according to claim 6, wherein said third frequency specified in said signal is chosen for inclusion in said signal in accordance with a pseudorandom selection scheme.
 8. The method according to claim 6, wherein said third frequency is selected from the group consisting of a next frequency and a next+1 frequency in said current hopping sequence.
 9. The method according to claim 1, further comprising: detecting the presence of a second signal by monitoring said second frequency; and determining a third frequency included in said current hopping sequence of said network based on information contained in said second signal.
 10. The method according to claim 1, further comprising monitoring during each of said defined frequency hops a corresponding one of said plurality of frequencies included in said initial frequency hopping sequence.
 11. The method according to claim 10, further comprising determining said first frequency for a plurality of said defined frequency hops by accessing an ordered list which specifies a plurality of alternate frequencies for each of said defined frequency hops.
 12. The method according to claim 1, further comprising monitoring at least one frequency included in said initial frequency hopping sequence for at least N+1 hops where N is the total number of hops in said initial frequency hopping sequence.
 13. The method according to claim 12, further comprising iteratively monitoring each frequency included in said initial frequency hopping sequence for at least N+1 hops until said signal is detected.
 14. The method according to claim 1, wherein a timing of said communication epoch is determined in accordance with a stable source of timing information, and said late entrant node accesses said stable source of timing information to determine a timing of said epoch.
 15. The method according to claim 14, wherein said stable source of timing information is a GPS signal.
 16. A communication transceiver for an ad-hoc radio network, comprising: a computer processing device configured to access from a memory at the communication transceiver an initial frequency hopping sequence for said ad-hoc radio network, said initial frequency hopping sequence specifying a plurality of frequencies, each respectively assigned to one of a plurality of defined frequency hops of said frequency hopping sequence, said initial frequency hopping sequence specifying a sequence of frequency hops used by the ad-hoc network at a time corresponding to initiation of the ad-hoc network; discover the current frequency hopping sequence by exclusively monitoring at least a first frequency specified by the initial frequency hopping sequence at a time after said initiation, when the initial frequency hopping sequence has migrated in a non-deterministic way to a current frequency hopping sequence different from the initial frequency hopping sequence, and unknown by the late entrant node; if the presence of a signal transmitted by a participating node of said ad-hoc network is not detected on at least the first frequency as a result of the exclusive monitoring, iteratively monitor one or more alternate frequency values in accordance with an ordered list respectively specified for at least the first frequency until detecting the presence of the signal, said signal comprising (1) first information concerning at least one “next+M” hop frequency of the current hopping sequence of said ad-hoc radio network, where M is an integer having a value equal to or greater than zero, and (2) second information useful for determining a time when the next+M hop frequency will be used by nodes of said ad-hoc radio network; and determine a second frequency included in said current hopping sequence of said ad-hoc radio network based on information contained in said signal.
 17. The communication transceiver according to claim 16, wherein said ordered list specifies an order in which said alternate frequencies are to be monitored by said communication transceiver.
 18. The communication transceiver according to claim 16, wherein said computer processing device is further configured to cause said communication transceiver to monitor at least the first frequency specified in said initial hopping sequence for a defined frequency hop and, if said signal is not detected, then monitor at least one said alternate frequency for said defined frequency hop.
 19. The communication transceiver according to claim 18, wherein said computer processing device is further configured to cause said communication transceiver to monitor in a sequential order each of said alternate frequencies for said defined frequency hop until said signal is detected.
 20. The communication transceiver according to claim 19, wherein said sequential order is specified with said ordered list.
 21. The communication transceiver according to claim 16, wherein said computer processing device is further configured to cause said communication transceiver to continue to monitor said first frequency during at least a second communication epoch, and determines a third frequency included in said current hopping sequence of said network based on information contained in said signal.
 22. The communication transceiver according to claim 21, wherein said third frequency specified in said signal is determined for inclusion in said signal in accordance with a pseudorandom selection scheme.
 23. The communication transceiver according to claim 21, wherein said third frequency is selected from the group consisting of a next frequency and a next+1 frequency in said current hopping sequence.
 24. The communication transceiver according to claim 16, wherein said computer processing device is further configured to: detect the presence of a second signal by monitoring said second frequency; and determine a third frequency included in said current hopping sequence of said network based on information contained in said second signal.
 25. The communication transceiver according to claim 16, wherein said computer processing device is further configured to cause said communication transceiver to monitor during each of said defined frequency hops a corresponding one of said plurality of frequencies included in said initial frequency hopping sequence.
 26. The communication transceiver according to claim 25, wherein said computer processing device is further configured to determine said first frequency for a plurality of said defined frequency hops by accessing an ordered list which specifies a plurality of alternate frequencies for each of said defined frequency hops.
 27. The communication transceiver according to claim 16, wherein said computer processing device is further configured to cause said communication transceiver to monitor at least one frequency included in said initial frequency hopping sequence for at least N+1 hops where N is the total number of hops in said initial frequency hopping sequence.
 28. The communication transceiver according to claim 16, wherein said computer processing device is further configured to cause said communication transceiver to iteratively monitor each frequency included in said initial frequency hopping sequence for at least N+1 hops until said signal is detected.
 29. The communication transceiver according to claim 16, wherein said computer processing device is further configured to determine a timing of said communication epoch in accordance with a stable source of timing information.
 30. The communication transceiver according to claim 29, wherein said stable source of timing information is a GPS signal. 